The present invention relates generally to certain compounds having optical properties. A specific embodiment of the present invention relates generally to high performance laser sources that use nonlinear optical processes to convert laser light to greater frequencies. More particularly, embodiments of the present invention provide a specific compound comprising MAl3B4O12 where M is one or a plurality of the elements {Sc, La, Y, or Lu}. In a particular embodiment, a laser apparatus is provided that produces coherent CW UV light with wavelengths that range between 190 and 350 nm and uses a device fabricated from materials described herein that are suitable for generation of electromagnetic radiation having a wavelength of 350 nm and less. However, it will be appreciated that the present invention has a much broader range of applicability.
Continuous wave (CW) ultraviolet (UV) lasers that emit light in the wavelength range 190-350 nm are in increasing demand for many industrial and research applications. Although CW lasers producing greater than 1 W of light in the 190-350 nm wavelength range have been constructed in research laboratories, such lasers have limited operating lifetimes and have not been significantly transformed into commercial products. In order to reach the UV wavelength spectral range, a source laser, emitting coherent radiation at a near-infrared (NIR) or visible wavelength, is converted to lower wavelengths (higher frequencies) by passing through one or more stages of frequency conversion to achieve second harmonic generation (SHG) and/or sum frequency generation (SFG). SHG stages convert a portion of the input light into light at a frequency of twice the input light (a wavelength of half of the input light). SFG stages take input light at two different frequencies and convert some of this input light into light at a frequency that is the sum of the input frequencies.
The SHG and SFG frequency conversion stages are created by using special nonlinear optical (NLO) materials that generate higher frequency light by processes that are now satisfactorily understood and that are categorized under the topic “nonlinear optics.” In many cases the NLO device is a single crystal of a nonlinear optical material that has been engineered to operate for a select purpose. In an SHG stage, light at the fundamental frequency (FF) propagates through the NLO device, and some significant fraction of this light is converted to light at the second harmonic (SH). The SH light is generated at different positions along the length of the NLO device, and propagates forward with propagation vector kSH. In order for the frequency conversion to be at all efficient, the propagating fundamental light must maintain an aligned in optical phase throughout the length of the NLO device with the SH light that is being locally generated. This requirement is called phase-matching. The phase of the newly-generated SH light is π/2 plus twice the phase of the FF light that has propagated up to the current position. Thus the phase-matching requirement is really a condition on the propagation constants for the FF and SH light. The condition for perfect phase-matching for a common type of SHG known as Type I SHG is:kSH=2kFF.  (1)
For SFG and for a less common type of SHG known as Type II SHG, the perfect phase-matching condition isk3=k1+k2,  (2)where the angular frequencies are related by ω3=ω1+ω2 and the propagation vectors may be non-collinear. A number of strategies for phase-matching are known in the art, including critical phase-matching, non-critical phase-matching (NCPM), and quasi-phase-matching (QPM).
Under operating conditions, the efficiency at which a NLO device converts light to the target frequency increases when the electric field strength of light at any of the involved wavelengths is increased. For SHG, the local power conversion rate for non-depleted conversion is proportional to the product of the intensity at the fundamental frequency and the square root of the intensity at the second harmonic:dISH(x)/dz∝IFF(x)ISH(x)1/2.  (3)
Here z is the distance propagated through the NLO device, and x is the position vector (x, y, z). For SFG, the local power conversion rate for unsaturated conversion is proportional to the square root of the product of the intensities at all three frequencies involved:dI3(x)/dz∝[I1(x)I2(x)I3(x)]1/2.  (4)
The simplest design of an SFG or SHG stage is a single-pass stage, where each of the input beam paths passes once through the NLO device and in a single direction. The output beam path in a single pass stage exits the crystal once and does not reenter it. Available sources of CW light with sufficiently good beam quality for frequency conversion have output powers on the order of watts or tens of watts. The single-pass frequency conversion stage with these laser sources produces overall power conversion efficiencies that are usually unacceptably low, typically well below 1%. In order to increase the efficiency, the intensities on the right side of Equations (3-4) must be increased by means of one or more enhancement cavities. These resonant cavities “build up” the field strength of light at designed frequencies by coherently adding (interfering) light that has traversed the cavity different numbers of times. Many UV laser designs with one or more cavities have been built and many more imagined.
There exists only a select few nonlinear materials that can usefully convert light into the deep UV. The two commercial materials are beta-Barium Borate (β-BaB2O4 or BBO) and Cesium Lithium Borate (CsLiB6O10 or CLBO). CLBO is very hygroscopic and requires an environment purged with a dry gas for harmonic laser generation to endure. CLBO is susceptible to fracture during a bake-out procedure that is necessary each time the crystal is exposed to ambient conditions. As a result, warm-up and cool-down procedures are very slow, typically on the order of several hours. Even with the most elaborate environmental control systems, lasers that use CLBO for harmonic conversion have a very limited lifetime.
The conversion efficiency of an NLO crystal for a particular application is dependent on a number of factors that include, but are not limited to: the effective nonlinearity of the crystal (pm/V), birefringence (Δn, where n is a refractive index), phase-matching conditions (Type I, Type II, non-critical, quasi, or critical), angular acceptance angle (radian-cm), temperature acceptance (K-cm), walk-off (radian), temperature dependent change in refractive index (dn/dT), optical transparency range (nm), and the optical damage threshold (W/cm2). Desirable NLO crystals should posses an optimum combination of the aforementioned properties as defined by the specific application.
Borate crystals form a large group of inorganic NLO materials used in various applications, such as laser-based manufacturing, medicine, hardware and instrumentation, communications, and research studies. Beta barium borate (BBO: β-BaB2O4), lithium triborate (LBO: LiB3O5), and cesium lithium borate (CLBO: CsLiB6O10) are examples of borate-based NLO crystals developed in recent years that are being used widely as NLO devices, especially in high power applications. Select properties suitable for generation of laser light from the mid-infrared to the ultraviolet for these crystals are listed in Table 1.
TABLE 1Commercially Available NLO Materials and PropertiesPROPERTYBBOLBOCLBODeff (pm/V)2.20.80.9Optical Transmission (nm)190-3500160-2600180-2750Angular Acceptance (mrad-cm)0.86.50.6Temperature Acceptance (K-cm)557.52.5Walk-off Angle (deg.)30.61.8Damage Threshold (GW/cm2)51010Crystal Growth Propertiesflux orfluxcongruentcongruent
BBO has a favorable non-linearity (about 2.2 pm/V), transparency between 190 nm and 3500 nm, significant birefringence (necessary for phase matching), and a high damage threshold (5 GW/cm2, 1064 nm, 0.1 ns pulse width). However, its high birefringence creates a relatively small angular acceptance that can limit conversion efficiencies and laser beam quality. The crystal is relatively difficult to grow to large sizes and is somewhat hygroscopic
LBO exhibits optical transparency throughout the visible electromagnetic spectrum, extending well into the ultraviolet (absorption edge.congruent. 160 nm), and possesses a high damage threshold (10 GW/cm2, 1064 nm, 0.1 ns pulse width). However, it has insufficient intrinsic birefringence for phase matching to generate deep UV radiation. Furthermore, LBO melts incongruently and must be prepared by flux-assisted crystal growth methods. This limits production efficiency that leads to small crystals and higher production costs.
CLBO appears capable of producing UV light due to a combination of high nonlinearity and sufficient birefringence. The crystal can also be manufactured to relatively large dimensions. However, the crystal usually is exceedingly moisture sensitive and often invariably sorbs water from the air; hence, extreme care usually must be taken to manage environmental moisture to prevent hydration stresses and possible crystal destruction.
In 1981 a crystal called NYAB [(Nd,Y)Al3B4O12] was reported in the USSR. A laser self-frequency-doubling effect from 1320 nm to 660 nm was realized in a Nd0.2Y0.8Al3B4O12 crystal, but it was found that intrinsic crystal absorption at the second harmonic limited practical use of laser self-frequency-doubling from 1060 nm to 530 nm.
Years later several institutes in China succeeded in improving the crystal growing process and obtained NYAB crystals of good optical quality and reasonable size. Lu et al. developed a multi-functional crystal (Nd,Y)Al3B4O12 with effective laser self-frequency-doubling conversion. The Nd3+ doped laser gain crystal was pumped with a dye laser, with laser emission at 1060 nm that was then converted to 530 nm within itself NYAB has since been used as a research crystal that often is useful only in the visible spectrum. Recent work with Yb-doped YAB as a self-doubling laser gain material follows the same path as NYAB with small alterations in operational laser efficiency and wavelengths. Laser light is generated within the crystal and self-doubled into green 520 nm. Again, its operation and the historic method of preparation limit its use to the visible and infrared. Hence, it is highly desirable to improve techniques for this family of compounds that enable optical function down into the ultraviolet.
BBO is somewhat hygroscopic, though less so than CLBO, and water-soluble. BBO also commonly undergoes degradation over time when used to generate UV light in pulsed, single-pass generation and in CW generation in a resonant cavity. Thus, the most significant shortcoming of both BBO and CLBO is their proclivity to degrade over time. In operation, these NLO materials lose frequency conversion efficiency under normal conditions, as illustrated in FIG. 1.
In an effort to circumvent these deleterious effects, many commercial lasers that use BBO periodically or continuously raster the crystal in the laser beam so that yet undamaged regions of the NLO device are accessed. In one particular commercial 266 nm wavelength laser, shifting the crystal every 8 hours and total laser refurbishment every 3000 hours are necessary solely to accommodate its delicate NLO device. With each translation, the optical cavity requires alignment and optimization, and the interruptions greatly drive up the laser cost-of-ownership in manufacturing environments. The lack of long term reliability in UV laser systems using BBO and CLBO crystals emphasizes the need for laser systems that use a more robust material for the generation of UV light. Thus, there is a need in the art for materials and systems for direct frequency conversion with improved performance during extended use.